This kind of exhaust purifying device for an internal combustion engine is described in Patent Document 1, for example. Conventional exhaust purifying devices, including the one described in Patent Document 1, each include a catalyst capable of storing oxygen and located in an exhaust passage of the internal combustion engine and an oxygen sensor located downstream of the catalyst in an exhaust stream. Here, a commonly known oxygen sensor has a property to output approximately 0 V when the air-fuel ratio of an exhaust is leaner than the stoichiometric air-fuel ratio and to output approximately 1 V when the air-fuel ratio of the exhaust is richer than the stoichiometric air-fuel ratio.
Incidentally, the more the catalyst deteriorates, the more deceased becomes the oxygen-storing capability of the catalyst, or the maximum oxygen storage amount. Under such circumstances, a conventionally used arrangement is to calculate the maximum oxygen storage amount of the catalyst to enable recognition of the degree of degradation of the catalyst on the basis of the maximum oxygen storage amount in the below-described manner. Specifically, the conventional arrangement is to perform active control of the air-fuel ratio to forcibly alter the air-fuel ratio of the exhaust that flows into the catalyst and calculate the maximum oxygen storage amount of the catalyst on the basis of engine operating conditions and changes in an output of the oxygen sensor.
Referring now to FIG. 10, active air-fuel ratio control operation commonly performed in the prior art is generally described hereunder.
FIG. 10 is a timing chart, in which section (a) represents changes in the air-fuel ratio of the exhaust that flows into the catalyst, section (b) represents changes in an output voltage of the oxygen sensor, and section (c) represents changes in the amount of oxygen stored by the catalyst during execution of the active air-fuel ratio control operation commonly performed in the prior art.
As shown in FIG. 10, the air-fuel ratio of the exhaust that flows into the catalyst is forcibly made higher than the stoichiometric air-fuel ratio at point in time t31 in the beginning so that the catalyst stores oxygen. When it becomes impossible for the catalyst to store any more oxygen, lean exhaust flows out on a downstream side of the catalyst, so that the output of the oxygen sensor changes from 1 V, which corresponds to rich mixture, to 0 V, which corresponds to lean mixture. Then, at point in time t32 where the output of the oxygen sensor becomes equal to V2, which corresponds to the stoichiometric air-fuel ratio, the air-fuel ratio of the exhaust that flows into the catalyst is forcibly made lower than the stoichiometric air-fuel ratio, so that oxygen is released from the catalyst. When it becomes impossible for the catalyst to release any more oxygen, rich exhaust flows out on the downstream side of the catalyst, so that the output of the oxygen sensor changes from 0 V to 1 V, which is representative of a rich mixture. Then, at point in time t33, where the output of the oxygen sensor becomes equal to V2, the air-fuel ratio of the exhaust that flows into the catalyst is forcibly made higher than the stoichiometric air-fuel ratio, so that the catalyst stores oxygen. When it becomes impossible for the catalyst to store any more oxygen, the output of the oxygen sensor changes from 1 V to 0 V, which is representative of a lean mixture. Then, at point in time t34, the output of the oxygen sensor becomes equal to V2. Here, the amount of oxygen released from the catalyst during a time period from the point in time t32 to the point in time t33 corresponds to the maximum oxygen storage amount of the catalyst. It is therefore possible to calculate the amount of oxygen released from the catalyst per unit time on the basis of engine operating conditions, such as the amount of injected fuel and the air-fuel ratio of the exhaust, and calculate the maximum oxygen storage amount by integrating the amount of oxygen released per unit time over the aforementioned time period (t32 to t33). Also, the amount of oxygen that has flowed into the catalyst during a time period from the point in time t33 to the point in time t34 corresponds to the maximum oxygen storage amount of the catalyst. It is therefore possible to calculate the amount of oxygen that has flowed into the catalyst per unit time on the basis of the engine operating conditions and calculate the maximum oxygen storage amount by integrating the amount of oxygen that has flowed into the catalyst per unit time over the aforementioned time period (t33 to t34).
Incidentally, the maximum oxygen storage amount of the catalyst varies with the temperature of the catalyst, as well as with the degree of degradation thereof.
Referring now to FIG. 11, a relationship between the temperature of a catalyst and the maximum oxygen storage amount thereof is described, by way of example, with reference to two catalysts of which the degrees of degradation differ from each other.
FIG. 11 is a graph representing the relationship between the temperature of each catalyst and the maximum oxygen storage amount thereof, wherein a solid line indicates changes in the maximum oxygen storage amount of a catalyst of which the degree of degradation is low and an alternate long and short dashed line indicates changes in the maximum oxygen storage amount of another catalyst of which the degree of degradation is high.
As shown in FIG. 11, the maximum oxygen storage amount Cmax of a catalyst increases with an increase in temperature TC thereof. Also, the maximum oxygen storage amount Cmax of a catalyst decreases with an increase in the degree of degradation thereof as mentioned earlier. Also, the difference between the maximum oxygen storage amount Cmax of the catalyst of which the degree of degradation is low and the maximum oxygen storage amount Cmax of the catalyst of which the degree of degradation is high becomes smaller as the temperature TC of the catalysts drops. As indicated by arrows in FIG. 11, however, there exist variations in calculated maximum oxygen storage amount (hereinafter referred to as actual maximum oxygen storage amount) CmaxA. Especially at low catalyst temperatures (T1<catalyst temperature TC<T2) where the actual maximum oxygen storage amount CmaxA is small, these variations greatly affect the actual maximum oxygen storage amount CmaxA, making it impossible to calculate the actual maximum oxygen storage amount CmaxA with high accuracy. Therefore, in order to calculate the actual maximum oxygen storage amount CmaxA with high accuracy, it is necessary to perform calculation when the temperatures of the catalyst is high (T2<catalyst temperature TC<T3). In this case, however, opportunities for calculating the actual maximum oxygen storage amount CmaxA are limited to occasions where the temperatures of the catalyst is high and, thus, there arises a problem that the catalyst can be examined for degradation thereof on limited occasions only.
In contrast, according to a technique described in Patent Document 1, the maximum oxygen storage amount increases proportionally with an increase in the temperature TC of the catalyst in a temperature range in which the catalyst is activated to a certain extent, in a range where the temperature TC of the catalyst is from T1 to T4 in the example of FIG. 11. Accordingly, focusing on the fact that the relationship between the temperature of the catalyst and the maximum oxygen storage amount thereof can be approximated by a linear equation and the gradient of the linear equation differs with the degree of degradation of the catalyst, an attempt is made to provide increased opportunities for calculating the actual maximum oxygen storage amount CmaxA in the below-described manner.
Referring now to FIG. 12, a relationship between the temperature of a catalyst and the maximum oxygen storage amount thereof as well as the gradient of a linear equation are described, by way of example, with reference to five catalysts of which the degrees of degradation differ from one another.
FIG. 12 is a graph representing the relationship between the temperature of each catalyst and the maximum oxygen storage amount thereof by a linear equation. In FIG. 12, a solid line L1 indicates changes in the maximum oxygen storage amount Cmax of a catalyst which is not degraded at all. Also, a short dashed line L2, an alternate long and short dashed line L3 and an alternate long and two short dashed line L4 indicate changes in the maximum oxygen-storing amounts Cmax of catalysts in the order of increasing degrees of degradation. Further, a long dashed line L5 indicates changes in the maximum oxygen storage amount Cmax of a catalyst with a maximum degree of degradation.
As indicated by the solid line L1 in FIG. 12, the maximum oxygen storage amount Cmax of the catalyst that is not degraded at all does not depend on the temperature TC of the catalyst and, thus, gradient G1 of the maximum oxygen storage amount Cmax with respect to the temperature TC of the catalyst becomes approximately 0. Also, as indicated by the short dashed line L2 and the alternate long and short dashed line L3 in FIG. 12, the more the degree of degradation increases from a state in which the catalyst is not degraded at all, the more gradients G2, G3 increase (G1<G2<G3). Further, as indicated by the alternate long and two short dashed line L4 and the long dashed line L5 in FIG. 12, the more the degree of degradation increases after having increased to a certain extent, the more gradient G4 decreases (G3>G4). Additionally, the maximum oxygen storage amount Cmax of the catalyst of which the degree of degradation is maximized does not depend on the temperature TC of the catalyst and, thus, gradient G5 of the maximum oxygen storage amount Cmax becomes approximately 0.
Taking the above in consideration, a function satisfied by the temperature TC of the catalyst and the actual maximum oxygen storage amount CmaxA thereof indicated in FIG. 12, specifically, the gradient of the linear equation, is determined in advance for each degree of degradation of the catalyst, and relationships between the temperature TC of the catalyst and the actual maximum oxygen storage amount CmaxA and gradients G of linear equations that specify degrees of degradation of the catalyst corresponding to these relationships are stored in storage means of a control unit. The control unit learns the gradient G of the linear equation corresponding to the degree of degradation of the catalyst on the basis of the actual maximum oxygen storage amount CmaxA calculated when the temperature TC of the catalyst is within a learning temperature range (T2<catalyst temperature TC<T3 in the example of FIG. 11) and the temperature TC of the catalyst during a period of calculation of the actual maximum oxygen storage amount CmaxA so that the actual maximum oxygen storage amount CmaxA will not be calculated at the same value despite different degrees of degradation. Then, when the actual maximum oxygen storage amount CmaxA of the catalyst is calculated anew, the calculated actual maximum oxygen storage amount CmaxA is corrected on the basis of the temperature TC of the catalyst during the same period of calculation, a reference temperature TCb (T2<TCb<T3), equation (1) below which is a linear equation, and the already learned gradient G of the linear equation. In this way, the corrected maximum oxygen storage amount Cmaxnrml, which is the maximum oxygen storage amount that would be achieved if the temperature TC of the catalyst has remained at the reference temperature TCb during the period of calculation, is calculated.Cmaxnrml=CmaxA+G(TCb−TC)  (1)
Referring now to FIG. 13, an example of how the corrected maximum oxygen storage amount Cmaxnrml is calculated is described.
FIG. 13 is a graph representing a relationship among the temperature of a catalyst, the actual maximum oxygen storage amount and the corrected maximum oxygen storage amount. In FIG. 13, a solid line indicates the actual maximum oxygen storage amount CmaxA of a catalyst of which the degree of degradation is low and an alternate long and short dashed line indicates the corrected maximum oxygen storage amount Cmaxnrml of the catalyst. A dashed line indicates the actual maximum oxygen storage amount CmaxA and the corrected maximum oxygen storage amount Cmaxnrml of a catalyst of which the degree of degradation is maximum.
As is obvious from FIG. 13, the corrected maximum oxygen storage amount Cmaxnrml becomes equal to the value of the actual maximum oxygen storage amount CmaxA at the reference temperature TCb, because the actual maximum oxygen storage amount CmaxA is corrected regardless of the temperature TC of the catalyst. This arrangement makes it unnecessary to prepare determination values for individual temperatures of the catalyst and simplifies the determination process performed in a configuration for determining the degree of degradation of the catalyst on the basis of a comparison between the maximum oxygen storage amount of the catalyst and the determination value, for example.